1. Field of the Invention
The present invention relates to a micromechanical sensor device and a corresponding manufacturing method.
2. Description of the Related Art
Although any micromechanical components are applicable, the present invention and its underlying object to be achieved are explained with reference to components which include inertial sensors based on silicon.
Micromechanical sensor devices for measuring acceleration, rotation rate, magnetic field, and pressure, for example, are generally known, and are mass-produced for various applications in the automotive and consumer sectors. In particular the miniaturization of components, functional integration, and effective cost reduction are trends in consumer electronics.
Nowadays, acceleration sensors and rotation rate sensors, as well as acceleration sensors and magnetic field sensors, are already manufactured as combination sensors (6d), and in addition there are first 9d modules, in which in each case 3-axis acceleration sensors, rotation rate sensors, and magnetic field sensors are combined into a single sensor device.
At the present time, mold packages dominate the market for inertial sensors; in mold packages, the silicon chips, for example a MEMS chip and an evaluation ASIC chip, are glued to a shared substrate, connected to one another and to external contacts via wire bonds, and subsequently coated with a plastic compound. So-called chip scale packages are of major importance for the future miniaturization of MEMS sensor devices and MEMS actuator devices. A plastic outer package is completely dispensed with in these components. Instead, the silicon chips are soldered directly to the application circuit boards via flip chip technologies. Such components are sometimes also often referred to as bare die structures. With regard to the footprint and possibly also the installation height, they have advantages over comparable products in mold packages.
One of the great challenges of chip scale packages for MEMS sensor devices is the control of stress effects. Due to the direct flip chip installation of the silicon chips on the application circuit boards, deformations are generally coupled more directly and more strongly into the MEMS chip than in mold packages, in which the introduction of stress is imparted via adhesives and molding compounds, and therefore occurs with alleviation.
Methods of so-called vertical integration, hybrid integration, or 3D integration are known, for example from U.S. Pat. No. 7,250,353 B2 or U.S. Pat. No. 7,442,570 B2, in which at least one MEMS wafer and one evaluation ASIC wafer are mechanically and electrically connected to one another via wafer bonding processes. These vertical integration methods in combination with silicon vias and flip chip technologies are particularly attractive, for which reason the external contacting may take place as a bare die module or a chip scale package, and thus without plastic outer packaging, as known from US Patent Application Publication 2012/0049299 A1 or US Patent Application Publication 2012/0235251 A1, for example.
US Patent Application Publication 2013/0001710 A1 provides a method and a system for forming a MEMS sensor device, in which a handling wafer is bonded to a MEMS wafer via a dielectric layer. After structuring the MEMS wafer to form the micromechanical sensor device, a CMOS wafer is bonded to the MEMS wafer, which includes the sensor device. At the end of the process, the handling wafer may be further processed by etching or back-grinding, if necessary.
FIG. 4 shows a schematic cross-sectional view for explaining the object to be achieved in an example of a micromechanical sensor device.
In FIG. 4, reference numeral 9 denotes a MEMS substrate, for example a chip substrate, which includes a silicon base substrate 13, a first insulating layer 14 which is applied thereto and structured, a first micromechanical functional layer 16 which is applied thereto and structured, and a second insulating layer 15 which is applied on top and structured. A second micromechanical functional layer 17 is deposited on top of second insulating layer 15 and structured. Insulating layers 14, 15 are made of silicon dioxide, for example, whereas first and second micromechanical functional layers 16, 17 are made of polysilicon. In this example, first, thinner micromechanical functional layer 16 made of polysilicon is used primarily as a conductor level including conductor sections LB, while movable micromechanical sensor structures MS for acceleration sensors, rotation rate sensors, or magnetic sensors, for example, are formed in second, thicker micromechanical functional layer 17.
Sensor structure MS, illustrated as an example, is connected to second insulating layer 15 or to first micromechanical functional layer 16 via rigid anchoring areas 17a, 17b. 
MEMS substrate 9 may contain additional micromechanical functional layers and insulating layers. The micromechanical functional layers may also be applied by wafer bonding processes and subsequent back-grinding.
Reference numeral 10 denotes an ASIC substrate having a front side VSa and a rear side RSa, for example likewise a chip substrate, which is preferably manufactured in a CMOS process. The ASIC substrate is made up of a base silicon substrate 18, doped semiconductor layers 19 for implementing integrated electrical circuits, and a rewiring element 20 which is formed on front side VSa of ASIC substrate 10 and which includes a plurality of stacked conductor levels LB1, LB0, contact plugs KS for electrically connecting conductor levels LB0, LB1 and for external electrical connection, and a plurality of insulating layers I which electrically insulate the conductor levels and their surroundings.
A via DK connects front side VSa of ASIC substrate 10 to its rear side RSa. An additional insulating layer 27 is deposited on rear side RSa, and includes rewiring lines 28a, 28b embedded therein which are used for electrical contacting. This electrical contacting and mechanical attachment to a carrier substrate 30 takes place with the aid of solder balls B1, B2. Carrier substrate 30 likewise includes conductors 30a, 30b for the electrical connection.
MEMS substrate 9 and ASIC substrate 10 are joined together via a metallic bonding process, for example a wafer bonding process, for example via eutectic bonding of aluminum with germanium. An uppermost aluminum conductor level is utilized as a bond surface on ASIC substrate 10, for example, and germanium is deposited on second micromechanical functional layer 17 of MEMS substrate 9 as the uppermost layer. The two substrates are then pressed together at temperatures above 430° C. with sufficient pressure so that a eutectic liquid phase results. Bond connection 50 made of AlGe (aluminum-germanium) on the one hand effectuates hermetic encapsulation of movable sensor structure MS in a cavity K with the aid of a circumferential bond frame 51, and on the other hand makes it possible for anchoring area 17b of second micromechanical functional layer 17 to have an electrical contact area 52 to ASIC substrate 10. Other metallic bonding processes, for example copper-tin bonding or thermocompression bonding, are likewise conceivable in principle.
To establish a stable mechanical connection between MEMS substrate 9 and ASIC substrate 10, a relatively wide circumferential bond frame 51 is usually implemented. Movable sensor structure MS is situated preferably symmetrically within this bond frame 51 in order to compensate for external stress effects.
Electrical contact areas 52 formed from bond connection 50 are usually implemented in the interior as very small contacts. Since a very large number of contacts are required, it is not possible to provide these contacts with a very large, and thus mechanically stable, design.
Electrical contact areas 52 may be placed either very close to bond frame 51 or in the middle of the chip. In a location close to the bond frame, the electrical contact areas are mechanically supported during the bonding process due to their immediate proximity to wide bond frame 51. However, in the separation process, usually a sawing process, contact areas 52 are very close to the cutting line, and experience vibrations which may emanate from that location and damage contact areas 52.
In the middle of the chip, contact areas 52 experience high mechanical stress during the bonding process, and may be damaged. During use, high mechanical stress on contacts 52 may also result at that location when, as in the present example, the component is soldered to a carrier substrate 30, which may transmit stress effects to the component.
In FIG. 4, this type of bending stress V results in cracks RI in electrical contacts 52. Such bending stress may be caused by the stress on the circuit board when the circuit board is pressed into a terminal device, or due to differing thermal coefficients of expansion. In addition to the damage to contact areas 52, contact areas 52 also always cause an asymmetrical mechanical deformation between MEMS substrate 9 and ASIC substrate 10 within bond frame 51. The MEMS substrate is therefore very sensitive to external stress, which reduces the service life.